Candida albicans is an opportunistic pathogen capable of causing debilitating mucosal infections as well as life-threatening systemic infections. Individuals infected with Human Immunodeficiency Virus (HIV) are particularly susceptible to Candida infections, with 90% of this population experiencing oropharyngeal candidiasis at some stage of the disease. C. albicans is ubiquitously present in the natural microbiota of the body and its success as both a commensal and a pathogen rely on its ability to adapt to changes in host physiology. This proposal will establish the mechanism and role of sexual reproduction in increasing phenotypic diversity and promoting infection of diverse sites in the mammalian host. C. albicans was originally thought to be obligately asexual but an efficient and elaborate mating program has now been established. Mating in C. albicans is unique in that it is regulated by phenotypic switching;only cells that have switched to the opaque form can undergo sexual reproduction. In addition, meiosis has not been observed and instead cells undergo an alternative program of concerted chromosome loss to complete a parasexual mating cycle. Progeny from the parasexual cycle are recombinant strains that exhibit novel phenotypes and have the potential for increased virulence in the host. A set of genotyped recombinant strains will now be analyzed using both in vitro and in vivo assays. Many of the products of the parasexual cycle are aneuploid strains carrying extra copies of chromosomes. As aneuploidy is associated with increased drug resistance in C. albicans our studies will determine if parasexual processes preferentially generate drug resistant strains. Sexual reproduction in C. albicans was thought to occur exclusively between a and 1 mating partners, yet our experiments have uncovered a novel mechanism for same-sex mating. Self-mating is promoted by autocrine pheromone signaling and the mechanism(s) of signaling will be established by genetic approaches. These studies also have potential implications for mechanisms of sexual reproduction in related Candida species (e.g. Candida parapsilosis) that propagate exclusively as unisexual populations. Our studies will also determine if the pheromone-signaling pathway enhances biofilm formation in C. albicans. Biofilm development is a key step in the colonization of implanted medical devices and enables growth of the pathogen prior to further invasion of the host. Experiments indicate the pheromone-signaling pathway promotes biofilm formation even in non-mating cells, and this hypothesis will be tested here. C. albicans is the most prevalent fungal pathogen of immunocompromised individuals yet the mechanisms promoting pathogenesis are poorly defined. We will determine the role that sexual reproduction plays in generating more virulent forms of the organism. In addition, we will establish if the machinery involved in sexual reproduction enhances pathogenesis by promoting biofilm development. PUBLIC HEALTH RELEVANCE: Candida albicans is the most common fungal pathogen in humans, causing both debilitating mucosal infections and life-threatening systemic infections, particularly in immunocompromised individuals such as those with HIV. Our research focuses on: (1) An understanding of the C. albicans mating cycle and how it leads to recombinant strains with altered virulence and/or drug resistance, and (2) The potential for components of the mating circuitry to enhance biofilm formation, a process critical for medical device-associated infections.